In feedback control of an air-fuel ratio of a vehicle engine, the air-fuel ratio oscillates about a target value due to factors such as the response speed of sensors used to detect the air-fuel ratio. This oscillation leads to a variation of engine torque, and if the frequency of the air-fuel ratio oscillation coincides with the natural vibration frequency of the drive system from the transmission to the vehicle wheels, resonance occurs and causes the vehicle to vibrate.
To prevent this vibration, Tokkai Sho 64-36941 published by the Japanese Patent Office in 1989 discloses a technique whereby, when the oscillation frequency of the air-fuel ratio coincides with the natural vibration frequency of the drive system, a control constant of the air-fuel ratio feedback control is altered.
Since the natural vibration frequency of the drive system is different according to the shift position of the transmission, this control constant is altered according to the shift position of the transmission during air-fuel ratio feedback control by comparing the corresponding natural frequency with the air-fuel ratio oscillation frequency.
However in the high load region of the engine where generated drive force is quite sensitive to the variation of air-fuel ratio, if the air-fuel ratio fluctuation frequency decreases due to alteration of the control constant, i.e. if the period of the air-fuel ratio oscillation increases, an air-fuel ratio which is leaner than a target air-fuel ratio predominates for a longer period of time. This leads to a deficiency of drive force and has an undesirable effect on the driving performance of the vehicle.
Tokkai Hei 7-269398 published by the Japanese Patent Office in 1995 discloses another technique whereby a target frequency different from that of the natural vibration frequency of the drive system is preset, and a control variable of air-fuel ratio feedback control is controlled so that the air-fuel ratio oscillation frequency coincides with this target frequency.
However according to this prior art technique, as the target frequency during steady state engine running conditions is set to a lower frequency band than the natural vibration frequency of the drive system, the control still causes the vehicle to be driven at a lean air-fuel ratio for long periods of time.